Bipolar junction transistors include emitter, collector and base regions. A biasing voltage applied between a base contact and an emitter contact allows control of collector current. Bipolar junction transistors can be employed in applications such as switches or amplifiers. Depending on the doping types associated with the base, emitter and collector regions, bipolar junction transistors may operate as p-n-p transistors or n-p-n transistors. A heterojunction bipolar transistor, which is a type of bipolar junction transistor, operates based on a bandgap difference between the emitter and base. Significant current gains can be obtained through the use of heterojunction bipolar transistors.
Some types of field effect transistors (FETs) have three-dimensional, non-planar configurations including fin-like structures extending above substrates. Such field effect transistors are referred to as FinFETs. The substrates may include semiconductor on insulator (SOI) substrates or bulk semiconductor substrates. Silicon fins are formed in some FinFETs on substrates via known technology such as sidewall image transfer (SIT). FinFET structures including SOI substrates can be formed, in part, by selectively etching the crystalline silicon layers down to the oxide or other insulating layers thereof following photolithography. Active fin heights are set by SOI thickness when employing SOI substrates. In bulk FinFETs, active fin height is set by oxide thickness and etched fin height. The gates of FinFETs can be formed using a “gate-first” process wherein a gate stack and spacers are formed prior to selective epitaxial growth wherein source and drain regions are enlarged. A “gate-last” process may alternatively be employed wherein the source/drain regions are formed immediately following fin patterning. Gate-last procedures can involve making a dummy gate, fabricating other elements of the transistor, removing the dummy gate, and replacing the removed dummy gate with actual gate materials.